Armies begin suffering minor attrition when they are no longer in or adjacent to their realm. The monthly percentage becomes more severe the longer they have been out of supply. This makes it dangerous to attack a distant enemy if you must march over neutral or hostile land.

Armies aboard ships do not suffer attrition, but time aboard ships is considered time out of supply. This reduces the effectiveness of unloading ships onto neutral lands to prevent the need to have an amphibious assault battle.

You are always in supply within your own realm, even when attacking a fellow vassal. Occupying a holding extends supply to neighboring provinces.